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SOIL
ANALYSIS TERMS
Soil pH
The soil pH measures active soil acidity or alkalinity. A pH of 7.0 is
neutral. Values lower than 7.0 are acid; values higher are alkaline. Usually
the most desirable pH range for mineral soils is 6.0 to 7.0 and for organic
soils 5.0 to 5.5. The soil pH is the value that should be maintained in the
pH range most desirable for the crop to be grown.
Buffer pH
This is an index value used for determining the amount of lime to apply
on acid soils to bring the pH to the desired pH for the crop to be grown.
The lower the buffer pH reading the higher the lime requirement.
Phosphorus
The phosphorus test measures that phosphorus that should be available to
the plant. The optimum level will vary with crop, yield and soil conditions,
but for most field crops a medium to optimum rating is adequate. For soils
with pH above 7.3 the sodium bicarbonate test will determine the available
P.
Potassium
This test measures available potassium. The optimum level will vary with
crop, yield, soil type, soil physical condition, and other soil related
factors. Generally higher levels of potassium are needed on soils high in
clay and organic matter versus soils, which are sandy and low in organic
matter. Optimum levels for light-colored, coarse-textured soils may range
from 90 to 125 ppm (180 to 250 lbs/ac). On dark-colored heavy-textured soils
levels ranging from 125 to 200 ppm (250 to 400 lbs/ac) may be required.
Calcium
Primarily soil type, drainage, liming and cropping practices affect the
levels of calcium found in the soil. Calcium is closely related to soil pH.
Calcium deficiencies are rare when soil pH is adequate. The level for
calcium will vary with soil type, but optimum ranges are normally in the 65%
to 75% cation saturation range.
Magnesium
The same factors, which affect calcium levels in the soil, also
influence magnesium levels except magnesium deficiencies are more common.
Adequate magnesium levels range from 30 to 70 ppm (60 to 140 lbs/ac). The
cation saturation for magnesium should be 10 to 15%.
Sulphur
The soil test measures sulfate-sulfur. This is a readily available form
preferred by most plants. Soil test levels should be maintained in the
optimum range. It's important that other soil factors, including organic
matter content, soil texture and drainage be taken into consideration when
interpreting sulfur soil test and predicting crop response.
Boron
The readily soluble boron is extracted from the soil. Boron will most
likely be deficient in sandy soils, low in organic matter with adequate
rainfall. Soil pH, organic matter level and texture should be considered in
interpreting the boron test, as well as the crop to be grown.
Copper
Copper is most likely to be deficient on low organic matter sandy soils,
or organic soils. The crop to be grown, soil texture, and organic matter
should be considered when interpreting copper tests. A rating of medium to
optimum should be maintained.
Iron
Soil pH is a very important factor in interpreting iron tests. In
addition, crops vary a great deal in sensitivity to iron deficiency.
Normally a medium level would be adequate for most soils. If iron is needed
it would be best applied foliar.
Manganese
Soil
pH is especially important in interpreting manganese test levels. In
addition, soil organic matter, crop and yield levels must be considered.
Manganese will work best if applied foliar or banded in the soil.
Zinc
Other factors, which should be considered in interpreting the zinc test,
include available phosphorus, pH, and crop and yield level. For crops that
have a good response to zinc, the soil test level should be optimum.
Sodium
Sodium is not an essential plant nutrient but is usually considered in light
of its effect on the physical condition of the soil. Soils high in
exchangeable sodium may cause adverse physical and chemical conditions to
develop in the soil. These conditions may prevent the growth of plants.
Reclamation of these soils involves the replacement of the exchangeable
sodium by calcium and the removal by leaching.
Soluble
Salts
Excessive concentration of various salts may develop in soils. This may be a
natural occurrence or it may result from irrigation, excessive fertilization
or contamination from various chemicals or industrial wastes. One effect of
high soil salt concentration is to produce water stress in a crop to where
plants may wilt or even die. The effect of salinity is negligible if the
reading is less than 1.0 mmhos/cm. Readings greater than 1.0 mmhos/cm may
affect salt sensitive plants and readings greater than 2.0 mmhos/cm may
require the planting of salt tolerant plants.
Organic
Matter and ENR (Estimated Nitrogen Release)
Percent organic matter is a measurement of the amount of plant and animal
residue in the soil. The color of the soil is usually closely related to its
organic matter content, with darker soils being higher in organic matter.
The organic matter serves as a reserve for many essential nutrients,
especially nitrogen. During the growing season, a part of this reserve
nitrogen is made available to the plant through bacterial activity. The ENR
is an estimate of the amount of nitrogen (lbs/acre) that will be released
over the season. In addition to organic matter level, this figure may be
influenced by seasonal variation in weather conditions as well as soil
physical conditions.
N03-N
(Nitrate Nitrogen)
Nitrate nitrogen is a measure of the nitrogen available to the plant in
nitrate form. In high rainfall areas, sandy soil types and areas with warm
winters, this measurement may be of limited value except at planting or side
dress time. In the areas with lower rainfall, the nitrate test may be very
beneficial.
Cation
Exchange Capacity (CEC)
Cation exchange capacity measures the soil's ability to hold nutrients such
as calcium, magnesium, and potassium, as well as other positively charged
ions such as sodium and hydrogen. The CEC of a soil is dependent upon the
amounts and types of clay minerals and organic matter present. The common
expression for CEC is in terms of milliequivalents per 100 grams (meq/100g)
of soil. The CEC of soil can range from less than 5 to 35 meq/100g for
agricultural type soils. Soils with high CEC will generally have higher
levels of clay and organic matter. For example, one would expect soil with a
silty clay loam texture to have a considerably higher CEC than a sandy loam
soil. Although high CEC soils can hold more nutrients, it doesn't
necessarily mean that they are more productive. Much depends on good soil
management.
Cation
Saturation
Cation saturation refers to the proportion of the CEC occupied by a given
cation (an ion with a positive charge such as calcium, magnesium or
potassium). The percentage saturation for each of the cations will usually
be within the following ranges:
Calcium: 40 to 80 percent
Magnesium: 10 to 40 percent
Potassium: 1 to 5 percent |
